Not all altcoins are centralized. Some altcoins are decentralized like Ethereum. The difference is you can't point out the actual person, people, or company that created Bitcoin but you can point out the people or company that own every other altcoin.
How are you defining the word "decentralized"? I'd argue that any coin that can go from PoW to PoS without anybody having a say in the matter is pretty damn centralized, unless I'm missing something obvious. Similarly, any altcoin that has a DAO or any kind of governance is centralized, even if mining or staking is decentralized by nature. Ugh. The state of altcoins has become atrocious IMO. I long for the simpler, easier days of 2016--but that's just me being retardedly sentimental about shitcoins.
Come to think of it, I was probably saying there were way too many shitcoins on the market back in 2016. And now? There are (too many)^3 if you get my meaning. It's amazing so many of them are doing well.